The compound 2,2-bis(4-hydroxyphenyl)propane, also called para, para-diphenylolpropane or Bisphenol A, is generally prepared by reacting phenol and acetone in the presence of an acidic catalyst such as hydrochloric acid along with a sulfur compound such as methyl mercaptan which acts as a co-catalyst. This method of preparing 2,2-bis(4-hydroxyphenyl)propane is disclosed in U.S. Pat. No. 2,730,552. However, the reaction between phenol and acetone to form 2,2-bis(4-hydroxyphenyl)propane also forms a number of byproducts including 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane also called ortho, para-diphenylolpropane, which is an isomer of 2,2-bis(4-hydroxyphenyl)propane. Further typical impurities include 2,2-bis(2-hydroxyphenyl)propane, higher condensation products such as trisphenols (condensation products of three moles of phenol and two moles of acetone) and chroman derivatives (internal condensation products of two moles of phenol and two moles of acetone), plus still higher condensation products in the form of resins and tars. As stated herein, "diphenylolpropane-type compounds" and "crude 2,2-bis(4-hydroxyphenyl)propane" refer to 2,2-bis(4-hydroxyphenyl)propane and the by-product derivatives indicated above.
In certain commercial applications of 2,2-bis(4-hydroxyphenyl)propane, such as in the production of polycarbonates, the 2,2-bis(4-hydroxyphenyl)propane must be of high purity, generally containing less than 0.3% weight impurities. Therefore, a number of methods to isolate 2,2-bis(4-hydroxyphenyl)propane in a substantially pure state have already been proposed.
One purification method, taught in East German Patent Specification 54,374, involves the crystallization of 2,2-bis(4-hydroxyphenyl)propane from a methanol solution by the addition of water or by the addition of halo-hydrocarbons, preferably dichloroethane, plus water, followed by washing the crystals with similar halohydrocarbons. However, this method is not attractive since it calls for the additional step of washing the crystals with halohydrocarbons.
Other relatively more simple recrystallization and extraction methods such as the method taught in U.S. Pat. No. 3,326,986 are known in the art. However, if these techniques are used to obtain 2,2-bis(4-hydroxyphenyl)propane in high purity, a number of operations, such as washing and flashing off solvent and/or wash liquid must be performed, which is a disadvantage, especially if 2,2-bis(4-hydroxyphenyl)propane is to be purified in large amounts.
Still further purification methods comprise recrystallizing the 2,2-bis(4-hydroxyphenol)propane from a suitable solvent such as benzene, toluene and methylene chloride as disclosed in British Patent 891,800 or diisopropyl ether as disclosed in British Patent 794,476.
Another purification method is taught in co-pending U.S. Ser. No. 511,980, filed Oct. 3, 1974, now U.S. Pat. No. 3,919,330 wherein 2,2-bis(4-hydroxyphenyl)propane is recovered from diphenylolpropane-type compounds by dissolving the diphenylolpropane-type compounds in ethylene glycol and adding a specified quantity of water to the ethylene glycol solution to precipitate the 2,2-bis(4-hydroxyphenyl)propane which is subsequently recovered by filtration or centrifugation. Although this method produces relatively high purity 2,2bis(4-hydroxyphenyl)propane it has been found that to obtain even higher purity 2,2-bis(4-hydroxyphenyl)propane it is necessary to subject the precipitated product to a subsequent crystallization, preferably from a toluene solution. When higher purity 2,2-bis(4-hydroxyphenyl)propane is necessary it would be desirable to restrict the handling of solid matter to one crystallization instead of two crystallizations.